In this topic, we will be discussing two types of channels:
color channels and
alpha channels. Specifically, we will discuss how to use a color
channel to create a selection and how to store a selection as an alpha channel.
Color channels contain the color and tonal information for the colors in an
color model. You can read more on the
Channels and Bit Depth page. Alpha channels, as discussed on this web
site, are used to store selection and layer mask information. You can read
about them on the
Alpha Channels page.
In Photoshop, both alpha and color channels are displayed in the Channels
panel. An RGB image, as shown in Figure 1, will have three individual
color channels and a composite channel. The composite channel is all the
individual color channels blended together. The individual channels
physically reside in your image file. The composite channel does not.
It is a virtual rendering of your image. A CMYK image will have four
individual channels and the composite. A grayscale image will have only
Figure 1. The Photoshop Channels panel
An individual color channel is represented as a black and white mask. This
mask indicates how much of the channel information should be used when rendering
a color. White areas in the mask use all the information. Black areas use
none. Gray areas use some of the information.
Like layer masks, channel masks are really 'hard coded' selections. This
topic describes how to turn this hard coded selection into an active selection.
Once it is an active selection, you can use it like any active selection created
by the marquee, lasso or magic wand tools. In the example below, we will
turn the active selection into a layer mask on an adjustment layer. Keep
in mind the channel itself is not deleted or altered in any way. We are
just taking the information in the channel and using it to create an active
Lets review how a mask works as a selection. White in a mask means the
area is fully selected. Black means the area is not selected. Gray
means it is partially selected. The lighter the gray the more the area is
selected. The darker the gray the less the area is selected.
When using a mask with an adjustment layer, those areas of a mask that are white
we be fully adjusted. The areas that are black in the mask will not be
adjusted at all. The gray areas will be adjusted in accordance with how
dark or light the gray is. The lighter the gray, the more the area will be
To learn how to turn a selection into an alpha channel, you will want to read
Follow these steps to use an individual color channel to create a selection.
- Activate the Channels panel.
- Click on each individual color channel until you find the one you want
to use. Make sure this is the only visible color channel.
- Click the Load Channel As Selection icon
at the bottom of the Channels panel.
- Click on the composite channel to reactivate all color channels.
- Click on the Layers tab to make the Layers panel the active panel.
- You will see the active selection in the document window.
To load the composite channel as a selection is easy. With the Layers panel
active, press Ctrl + Alt + ~ (Command + Option + ~) (~ is the tilde character).
Now that the easy part is done, lets discuss what is happening. When you
first use Ctrl + Alt + ~ (Command + Option + ~) on your image, you may think a
selection is being placed around individual colors. Actually, it is selecting
tonal values. If the tone is solid white, it is fully selected. If
the tone is solid black, it is not selected. Tones in-between are
partially selected. Light tones will be selected more than mid tones,
which will be selected more than dark tones.
My experience shows that it is the perceived luminance (or %K) that is being
selected. To learn more, read the
and Measured Luminance page.
Now that we have learned how to load a channel as a selection, lets use it in
a practical example.
In this example, we will use a previously color corrected image to isolate a
hard to select area and change its color. Specifically, we will isolate
the yellow leaves in Figure 2A and make them orange, as seen in Figure 2B.
We used this same image on the
to also change the leaves to orange. However, in that example we used
color ranges to identify and change the colors. In this example, we
will use an individual color channel to create a mask on a Hue/Saturation
Figure 2A. Before...
Figure 2B. and After
Below are the steps used to isolate the yellow leaves using a color channel,
use the selection as a layer mask in a Hue/Saturation adjustment layer, modify
the mask to our needs and change the color to orange.
- Click on the Channels tab to make the Channels panel the active
- Click on each individual color channel and find the channel where the
leaves are the whitest and easily distinguishable from their
- After looking at each individual color channel, the red channel best met
the criteria in step 2 for this image. Therefore, the Red channel was made
the only active channel.
- At the bottom of the Channels panel, click the Load Channel As
- Click the composite channel to reactivate all channels.
- Click the Layers tab to exit the Channels panel and activate the
- Click Layer > New Adjustment Layer and choose Hue/Saturation. Or, click
the Create New Fill or Adjustment Layer icon
at the bottom of the Layers panel.
- Name the layer and click OK. At this point, Photoshop will add the
adjustment layer to the document and create a layer mask based on the
selection. The Hue/Saturation dialog box will be open and the active
selection will be gone.
- Move the Hue slider until the desired result is achieved. In this case,
Hue was set to -15. (This means we took the existing color and replaced it
with the color that is 15° counterclockwise from it on the
color wheel.) At this point, do not be concerned if other parts of the
image are also changed.
- Click OK to close the dialog box.
- To isolate the change to the leaves, we will need to paint in the layer
mask. Alt + click (Option + click) the layer mask on the Hue/Saturation adjustment layer.
The layer mask will be loaded into the document window. It should look
identical to the red channel mask, as seen in Figure 3A.
- Using black paint
the Brush tool
and a soft edged brush, paint in the layer mask to cover up all areas except
the leaves. The result can be seen in Figure 3B. (You can download both
soft and hard edged brushes from the
- Alt + click (Option + click) the layer mask again to restore the image. The result is
shown in Figure 2B.
Figure 3A. The Red Channel Mask
Figure 3B. Red channel loaded into layer mask and painted black
The key points when using this technique are as follows.
- Use the channel where the target area is the whitest and easily
distinguishable from its surroundings.
- After the selection is used to create the layer mask, use black paint
and the Brush tool
to paint in the layer mask those areas you do not want affected by the
Cannot find an individual color channel where the area
you want is white? Try finding a channel where the area you want is
black, yet still distinguishable from its surroundings. Load this
channel as a selection and then click Select > Inverse to invert the
selection. Inverting the selection should be done prior to turning the
selection into a layer mask.